Heretofore, balanced safety valves or balanced relief valves have been provided in pressure relief systems. While relief valves of various types have proven effective in applications where the fluid product is a benign gas or liquid at ambient or near-ambient temperatures and low to moderate pressures, they are not suitable for use in processes that are chemically incompatible with elastomer-type O-ring seals, or that have extremely low service temperatures or high backpressures. Most O-rings are not suitable for effective sealing when exposed to temperatures below approximately −60° F., and those known for their greatest chemically resistance, such as those of the perfluoroelastomer family, are not suitable below 0 to −20° F. The bellows in a balanced bellows-style valve may not be sufficiently durable in high backpressure applications.
Pressure relief discharge pipelines are frequently vented directly to atmosphere, where a conventional unbalanced spring-operated safety valve may be used due to the lack of backpressure present in the discharge lines that could vary the set pressure of the valve. Where the pressure relief system is handling hazardous or expensive fluids, discharge directly to atmosphere is not practical and may be contrary to environmental regulations. In these cases, one or more pressure relief valves would be installed such that each of the respective discharge pipelines is connected to a common header or manifold that leads to a central collection system or is routed back to an earlier stage of the fluid processing stream. The discharge header or manifold commonly contains a constant or variable level of pressure that may be due to design conditions of the fluid collection system or an actual discharge from one or more of the connected pressure relief valves. Such mainfold pressure, referred to in the industry as “superimposed backpressure,” would exist all the way back to the discharge side of each of the connected pressure relief valves and exert a net force on the valve closure member (disc or spindle) that is proportional to the value of the pressure in combination with the net surface area of the valve closure member. That is, when superimposed backpressure is allowed to act on opposing closure member surfaces that are not of equal area, as in an unbalanced relief valve, the set pressure of the relief valve is caused to change. Variable set pressures at a pressure relief valve installation are generally undesirable as they may compromise the safety of the equipment (tank, pipe, pump, etc.) being protected and will otherwise cause inconsistent system operation.
In balanced pressure relief valves that rely on a metal bellows to achieve pressure balance of the main closure member, some designs place significant restrictions on the amount of allowable backpressure to which the bellows can be exposed, as the bellows tends to be of thin, light construction and may be easily subject to rupture.
What is needed, then, is a balanced safety relief valve in a pressure relief system for a pressure vessel or for gas or liquid product pipelines, more particularly an improved spring-operated safety relief valve balanced against the effects of backpressure, where the internal sealing configuration can withstand high system pressures and is chemically compatible with a wide variety of process fluids and low temperature conditions.